1. Field of the Invention
The present invention relates to a monoclonal antibody which specifically recognizes a native three-dimensional structure of an extracellular region of system ASC amino acid transporter 2 (hereinafter, referred to as “ASCT2”) and binds to the extracellular region, or an antibody fragment thereof; a hybridoma which produces the antibody; a DNA which encodes the antibody; a vector which contains the DNA; a transformant obtainable by introducing the vector; a process for producing an antibody or an antibody fragment thereof using the hybridoma or the transformant; and a therapeutic agent using the antibody or the antibody fragment thereof, and a diagnostic agent using the antibody or the antibody fragment thereof.
2. Brief Description of the Background Art
An ASCT2 polypeptide is a 10-times transmembrane protein consisting of 541 amino acids in full-length form, and functions as a transporter which transports neutral amino acids through a cell membrane depending on a sodium ion. The amino acid transporter is categorized into several systems, based on functional characteristics including substrate specificity and the like. ASCT2 belongs to System ASC and has functions of intracellular uptake of neutral amino acids such as L-alanine, L-serine, L-threonine, L-cysteine, and L-glutamine depending on a sodium ion (Non-Patent Document 1).
Further, ASCT2 is a viral cell-surface receptor which is shared by type D simian retrovirus and three type C viruses [feline endogenous virus (RD114), baboon endogenous virus, and avian reticuloendotheliosis virus] (Non-Patent Documents 2 and 3).
ASCT2 is also known as sodium-dependent neutral amino acid transporter type 2, adipocyte amino acid transporter, AAAT, neutral amino acid transporter B, ATB, amino acid transporter B0, ATB0, ATBO, baboon M7 virus receptor, M7V1, M7VS1, insulin-activated amino acid transporter, FLJ31068, RD114 virus receptor, RD114/simian type D retrovirus receptor, RDR, RDRC, or R16 (Non-patent Documents 1 and 3).
Furthermore, as another name, ASCT2 is known as Solute carrier family 1 (neutral amino acid transport), member 5, Solute carrier family 1 member 5, or SLC1A5 and belongs to SLC1 family.
With respect to the relationship between the development of cancer and the expression of ASCT2 or SLC1A5, it has been found that expression levels of three SLC1A5, SLC7A5 and SLC38A5 are significantly increased in cancerous tissues by studies using expressed sequence tag (EST) database (Non-Patent Document 4). Further, the expression of SLC1A5 is not recognized in a normal liver, but is recognized in clinical tissues of hepatocellular carcinoma or hepatoblastoma (Non-Patent Document 5). In addition, it has been found that the expression of SLC1A5 is higher in clinical tissues of poorly-differentiated astrocytoma or glioblastoma multiforme than that of normal tissues (Non-Patent Document 6).
Further, the expression of ASCT2 is detected in clinical tissues of colorectal cancer and prostate cancer by immunohistological staining or Western blotting, and patients who highly express ASCT2 have a poor outcome (Non-Patent Documents 7 and 8).
Furthermore, ASCT2 is responsible for the uptake of glutamine in several cell lines of colorectal cancer, liver cancer, breast cancer, astrocytoma and neuroblastoma (Non-Patent Documents 9 to 12), and the proliferation of cells is suppressed by competitively inhibiting intracellular uptake of glutamine using an alanine-serine-threonine mixture which is a substrate for ASCT2 (Non-Patent Document 9).
As regards an antibody which binds to ASCT2, a polyclonal antibody against partial peptides of the intracellular N-terminal or C-terminal of human ASCT2 is known (Non-Patent Documents 13, 14 and 15). Since all of these are antibodies which recognize an intracellular portion of ASCT2, they cannot bind to ASCT2 which is expressed on the cell membrane of cells. It is known that when an antibody binds to an antigen protein expressed on the cell membrane, a cellular cytotoxicity due to an effector activity of the antibody, such as antibody-dependent cellular cytotoxicity (hereinafter, referred to as “ADCC activity”) or complement-dependent cytotoxicity (hereinafter, referred to as “CDC activity”) is induced within the living body (Non-Patent Document 16). However, since these antibodies cannot bind to ASCT2 expressed on the cell membrane of cells, these cellular cytotoxicities due to the effector activity cannot be expected. Further, since all of these antibodies are antibodies which recognize an intracellular portion of ASCT2, they cannot inhibit intracellular uptake of amino acids by ASCT2 which expresses in living cells.
In addition, polyclonal antibodies which recognize human ASCT2 are commercially available from LifeSpan BioSciences (Catalog Numbers: LS-C16840 and LC-C31887), Avia Systems Biology (Catalog Number: ARP42247_T100), Santa Cruz Biotechnology (Catalog Numbers: sc-50698 and sc-50701), or Millipore (Catalog Number: AB5468).
However, it is not known a monoclonal antibody which specifically recognizes a native three-dimensional structure of an extracellular region of ASCT2 and binds to the extracellular region.    [Non-Patent Document 1] J. Biol. Chemistry, 271, 18657 (1996)    [Non-Patent Document 2] Proc. Natl. Acad. Sci. USA, 96, 2129 (1999)    [Non-Patent Document 3] J. Virology, 73, 4470 (1999)    [Non-Patent Document 4] Seminars in Cancer Biology, 15, 254 (2005)    [Non-Patent Document 5] Am. J. Physiol. Gastrointest. Liver Physiol., 283, G1062 (2002)    [Non-Patent Document 6] NeuroReport, 15, 575 (2004)    [Non-Patent Document 7] Anticancer Research, 22, 2555 (2002)    [Non-Patent Document 8] Anticancer Research, 23, 3413 (2003)    [Non-Patent Document 9] J. Surgical Research, 90, 149 (2000)    [Non-Patent Document 10] J. Cellular Physiology, 176, 166 (1998)    [Non-Patent Document 11] J. Neuroscience Research, 66, 959 (2001)    [Non-Patent Document 12] Am. J. Physiol. Cell Physiol., 282, C1246 (2002)    [Non-Patent Document 13] J. Gene Medicine, 6, 249 (2004)    [Non-Patent Document 14] Am. J. Physiol. Cell Physiol., 281, C963 (2001)    [Non-Patent Document 15] Biochem. J., 382, 27 (2004)    [Non-Patent Document 16] Cancer Res., 56, 1118 (1996)